1. Field of the Invention
The present invention relates to an on-vehicle equipment provided on a side of a vehicle in dedicated short-range communication in intelligent transport system, in particular, an on-vehicle equipment which can firmly fix a desirable receive channel with high efficiency.
2. Description of Background Art
Intelligent transport system (hereinbelow referred to as ITS) is to transmit and receive various data by communication between an on-road equipment provided on a side of road and an on-vehicle equipment provided on a side of vehicle in use of dedicated short-range communication (hereinbelow referred to as DSRC) for communicating only within a limited range on a road utilizing a radio wave in the microwave band.
In ITS, an electronic toll collection system (hereinbelow referred to as ETC) is included. It is a system for automatically collecting toll without stopping vehicles. The system enables an automatic toll payment procedure in use of the above-mentioned DSRC without temporarily stopping a vehicle when the vehicle passes through a tollgate in an expressway. In ETC, an on-vehicle equipment on the vehicle side communicates with on-road equipments, respectively provided in tollgates. Two frequencies F1 and F2 are alternately allocated to adjacent gates to avoid interference between the frequencies. These two frequencies F1 and F2 and a communication method are determined by the standard of DSRC.
According to DSRC, for example, a vehicle approaching a toll gate alternately switching over between the frequencies F1 and F2 as a local frequency because it is unknown whether the frequency of the toll gate is F1 or F2. The vehicle receives data, and selects the frequency by fixing the frequency when the data are normally received.
FIG. 9 is a block chart in a conventional on-vehicle equipment for DSRC used in ITS disclosed in, for example, JP-B-2947797. In FIG. 9, numerical reference 1 designates a receiving antenna for receiving radio wave sent out of the on-road equipment; numerical reference 2 designates a frequency converting unit for converting a frequency of the radio wave received by the receiving antenna 1 to a predetermined receiving frequency; numerical reference 3 designates a channel selecting filter for taking out only a predetermined channel from the signal of which frequency is converted by the frequency converter 2; numerical reference 4 designates a demodulator for digitizing an output from the channel selecting filter 3; numerical reference 5 designates a normal reception determining unit for judging whether or not the received data are normal using error check such as cyclic redundancy check (CRC); numerical reference 6 designates a phase locked loop (PLL) oscillating unit for producing a local frequency for the frequency converting unit 2; numerical reference 7 designates a frequency switching determining unit for determining the frequency from the received data and judging whether or not the local frequency outputted from the PLL oscillating unit 6 is fixed or switched over based on the received F1/F2 signal as the frequency signal of the judged frequency and selected F1/F2 signal as a selecting frequency signal representing a currently selected frequency; numerical reference 8 designates a timer for controlling a cycle of the selected frequency signal; and numerical reference 9 designates a frequency memory unit, in which the frequencies F1 and F2 sent out of the on-road equipments are memorized.
Next, an operation will be described. FIG. 10 is a flow chart showing an operation of the frequency switching determining unit in the conventional on-vehicle equipment for DSRC in the ITS. In Step S41 of FIG. 10, it is judged whether or not the timer 8 for controlling the output cycle of the selected F1/F2 signal is effective. If the timer is not effective, in Step S42, when the currently selected frequency is F1, it is switched to F2, and when the currently selected frequency is F2, it is switched to F1, and the selected F1/F2 signal is outputted. In Step S43, the timer 8 is set.
If the timer 8 is judged to be effective in Step S41, it is judged whether or not a radio wave sent out of the on-road equipment is normally received in Step S44. If the radio wave is normally received, in Step S45, the received frequency is determined. In Step S46, the received F1/F2 signal is outputted. In Step S47, the received F1/F2 signal is compared with the selected F1/F2 signal selected in Step S42. In Step S48, when the received F1/F2 signal is the same as the selected F1/F2 signal, a local frequency outputted from the PLL oscillating unit 6 is fixed. In Step S49, if the received F1/F2 signal is different from the selected F1/F2 signal, the local frequency outputted from the PLL oscillating unit 6 is switched over.
In the conventional DSRC of ITS, only ETC is available, whereby the two frequencies of the above-described two channels of F1 and F2 are used. However, in recent years, various applications to, for example, collection of charges in gas stations and drive-through shops and traffic information service are planned besides ETC. Accordingly, seven frequencies (seven channels) are allowed to use in accordance with the DSRC standard.
In the conventional on-vehicle equipment for DSRC, there is no measure to distinguish a frequency used in a communication area and it is impossible to select the frequency until a vehicle approaches to the communication area as described above. Therefore, the two frequencies are switched over by a predetermined time in order to select one. However, when the system is applied to an on-vehicle equipment for DSRC using all applications other than ETC, it is necessary to select the seven frequencies by an equal time period, whereby there is a problem that a substantially long time is required to select the frequencies.
FIG. 11 illustrates a procedure of the conventional on-vehicle equipment for DSRC, wherein the problems will be explained in reference of the figure. An entrance of a toll lane or the like ordinarily has two antennas, arranged in serial with respect to a traveling direction of vehicle, by each lane for the purpose of preventing erroneous detection of types of motor vehicle, and an identical frequency, e.g. F1, is sent. In FIG. 11, rectangles of solid line represent an outgoing signal from the first antenna, positioned on the near side of the traveling direction of vehicle, and rectangles of broken line represent an outgoing signal from the second antenna in the traveling direction of vehicle, wherein the outgoing signals from the first and second antennas are temporally separated.
As shown in FIG. 11, one frame is formed by two types of slots, namely frame control message slot (hereinbelow referred to as FCMS), in which frequency information is written, and message data slot (hereinbelow referred to as MDS), of which frequency information is blank. It is possible to select the frequency by receiving FCMS. The slot has a length of about 0.78 ms. However, as illustrated in FIG. 11, when the head of the first frame is not received, it is necessary to continue the receipt for approximately more than 5.46 ms until the tail of FCMS in the succeeding frame. By adding a frequency fix time of about 1.5 ms in PLL, it is necessary to search the frequency at an interval of more than about 7 ms.
Accordingly, when the conventional technique is applied to ITS using seven frequencies described above, it is necessary to provide a frequency search time utmost about 49 ms. Therefore, there are problems that the search time becomes very long and the ETC standard requiring that the frequency be completely selected within nine frames, i.e. about 21.1 mS, is not satisfied.